OAKLAND -- A former UC Berkeley graduate student instructor has been charged with two counts of possession of a controlled substance after police said he slipped a drug into a woman's drink and then sexually assaulted her.

Timothy Rodriguez, 40, was charged Friday by the Alameda County District Attorney's office. He was arrested Tuesday on suspicion of unlawful sexual penetration and possession of controlled substances. The district attorney's office, however, did not file sexual assault charges against him. A spokeswoman for the office declined to comment on the case, saying the investigation is ongoing.

UC Berkeley police say that on Jan. 25, Rodriguez met a 22-year-old college student at a bar near campus, where the two had drinks together. The woman reportedly left her drink unattended for a short period of time, then became intoxicated and ill after she returned, police said.

Rodriguez then took the woman to his home at University Village in Albany and assaulted her, police said. Following his arrest Tuesday, he was jailed, but posted $120,000 bail and was released.

Police advised bar patrons to never leave drinks unattended and to be wary of accepting drinks from strangers. They also encouraged using a buddy system while socializing.